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I HAVE studied the very penetrating component of cosmic radiation by means of a large vertical Wilson chamber actuated by counters, following the method of Blackett and Occhialini. The chamber is 55 cm. high and 15 cm. broad and is placed between the large poles of 75 cm. diameter of the electromagnet at Bellevue of the Paris Academy of Sciences. The magnetic field is 13,000 gauss over all the chamber. The experimental apparatus is shown in Fig. 1. A 7 cm. block of lead is placed between the counters. Rays such as AB were photographed. The cosmic rays photographed with this arrangement are those which are able to penetrate the lead block, that is, the more penetrative component of the spectrum alone is photographed.